With the technologies of image process and digital imaging running up, various kinds of advanced digital color copying apparatus arise. Those digital imaging apparatus can copy financial notes, securities, important certificates, classified documents and so on with a high fidelity. As a result, it is very difficult to distinguish the original from the copy from time to time. The copy machine is a typical example. Modern copy machines possess a high ability to make high quality copies with advanced functionality, some of which possess an intelligent editing capability and realize communication with other peers. Some advanced copy machines can so much as make a copy of banknote. Such copy machines can copy various documents with high quality, which greatly reduces the work load of transcription and improves efficiency. However, this ability of the copy machines produces a problem for the security of important documents, that is, classified documents might be easily copied during transmission so as to lose the security. Then the copy machine becomes a convenient tool for leaking or stealing secrets. In recent years, most of the intercepted classified documents by the customs are copies. It has clearly shown the seriousness of the problem. The same risk is also carried with advanced scanners, facsimile machines, digital cameras, as well as other imaging apparatus with high definition. To resolve this problem, in addition to reinforcing management and control of the classified materials, the copy protection technology is necessary to be developed for classified materials so as to restrain the high-fidelity image copying function furthest.
A method is disclosed in the prior art, in which specific papers with copy-protected shade is used to prevent the classified documents from being copied. The additional digital watermark information is embedded in the original image data. Specifically, a layer of copy-protected shade is pre-printed on the printing paper and the obscure words such as “Copy”, “VOID” and “Copy Prohibit” are pre-embedded in the shade. When the document is unauthorizedly copied, those words stand out so that the copy can be distinguished clearly. Psychologically speaking, this method provides anti-counterfeiting to a certain degree, for example, a copy of a ticket or important material is easily identified by an examiner. However, for those classified documents, the emphasis is on the leak of the document, other than the warning on the copy. Therefore, such method does not fundamentally prevent the document from being copied.
In another method of the prior art, the specific marks which are printed on the classified documents and indicate the confidentiality are examined so as to identify the documents to be prevented from being copied. It is common that a classified document is printed with such specific marks to indicate its confidentiality. If it is easy for identification that the input original is a specific document, the classified documents are copy-protected. However, if a classified document has specific classified marks such as “classified”, the document can be easily defeat by covering up those specific classified marks with a piece of paper. Therefore, this method still could not fully protect the classified documents.
Furthermore, there is another similar method: by using pattern matching, the input image data is compared with the pre-stored specific marks. The input image is concluded as a special document if the input image data matches a pre-stored specific mark. Therefore, the special documents, such as securities, currencies, and the like, can be recognized. This method requires pre-stored reference pattern data for the specific documents. However, in fact, it is very difficult to apply the reference pattern data of static data to a huge number of classified documents. Of course, other pattern matching solutions could be also used to protect classified documents. However, all the pattern matching solutions need a large hardware storage space for pre-storing reference patterns for basic units. The required hardware storage space increases with the increasing stored reference patterns for basic units. To prevent the classified documents from being copied at a turned angle, accordingly, it is necessary for the hardware storage to store the specific information of reference patterns at various angles for correctly identifying the documents with copy protection. Because the pattern matching technique is applied here, when the original protected document is enlarged or shrunk for copy, the hidden specific information cannot be identified correctly by using this method in many cases. Furthermore, the above method is very complicated, requires a large hardware storage space, and generally needs computer-accessible storage medium or special image-processing device to carry out. In such methods, received input images are obtained by a scanner, and then processed by specific devices. When an advanced copy machine is used to enlarge or shrink illegally classified documents for copying, those methods stated above become useless. Therefore, those methods still cannot fundamentally prevent the classified documents from being copied.